Pre-stressed concrete box-beams have been used since the 1950's to build short and medium span bridges. It is estimated that there are approximately 54,000 box beam bridges in service in the United States. Box-beams are popular because they can be used in multiple ways to quickly and safely construct bridges.
Box beams are typically manufactured by casting reinforced concrete around a foam core in a mold or formwork. Rebar is used to build a reinforcing “cage” around the foam core prior to pouring the concrete. The bottom part of the cage is placed first and then a layer of concrete is poured. The foam core is placed on the layer of concrete and the top of the cage is built up around the foam core. Concrete is poured to surround the foam core and the rest of the cage.
Because the foam core fills the central space of the box beam and is surrounded by concrete after the pour, it is not possible to do a post-cast inspection of the interior of the box-beam. It is also possible for the foam core to shift during the pour causing inconsistencies in concrete thickness in the webs and flanges, which change the strength of the beam. Further, the time involved in constructing and pouring the concrete in two stages can result in formation of a “cold joint” between the bottom and top parts of the beam, which are poured in two stages. A cold joint can form when a fluid concrete is poured over set or semi-set concrete. The interface where two different phases of concrete meet can form a cold joint, which can be weaker than other more consolidated areas of the concrete mass.
During construction, box beams are placed adjacent to one another with a small gap between called a shear key. The shear key can be filled with a cementitious grout material. Once the box-beams are set and grouted, a transverse post-tensioning (TPT) arrangement is used to apply force across the box-beams. This is followed by laying a 3″ to 6″ reinforced deck slab over the box-beams or an asphalt overlay for low-traffic bridges. When the box-beams, grouted-shear keys, TPT, and the deck slab are properly integrated, a completed bridge can perform as a monolithic structure.
It is normal for cracks to form in grout material between the box-beams. The box-beams can also develop small fractures or cracks. Water and other materials that seep into the cracks can be absorbed by the grout and the foam. This creates an internal environment that is moist and, over time, promotes degradation of the grout material and the box beam. Because the internal void cannot be inspected, the rate of degradation cannot be easily monitored.
Ultra-high performance concrete (UHPC) has become an important structural material. UHPC benefits from being a “minimum defect” material. That is, UHPC is a material that is less susceptible to the formation of defects such as micro-cracks and interconnected pores, and exhibits a maximum packing density. Several types of UHPC have been developed in different countries and by different manufacturers. The four main types of UHPC are compact reinforced composites (CRC), multi-scale cement composite (MSCC), and reactive powder cement (RPC). RPC is the most commonly available UHPC and one such product is currently marketed under the name Ductual® by Lafarge, Bouygues and Rhodia.
There is a need for a reinforced concrete beam that can be cast in a single concrete pour that still has an internal void to be used for inspecting the beam surfaces after casting. The ability to use UHPC as a grout material between the beams would also be an advantage.